1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus having the zoom lens, and is suitable for an image pickup apparatuses using a solid-state image pickup device such as video cameras, electronic still cameras, broadcasting cameras, and monitoring cameras or image pickup apparatus such as cameras using silver-halide film.
2. Description of the Related Art
Photographic optical systems used in image pickup apparatuses are required to be a zoom lens having a short entire lens length, being compact, having a wide field angle, and having a high zoom ratio.
In particular, being a zoom lens having a high resolving power in which chromatic aberration is adequately corrected in addition to single-color aberration such as spherical aberration and coma aberration is required.
A positive-lead type zoom lens including a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a rear lens group including one or more lens units in this order from an object side to an image side is known.
As a positive-lead type zoom lens, a zoom lens having a zoom ratio on the order of 10 in which an anomalous dispersion material is used for lenses of the first lens unit to adequately correct chromatic aberration is known (U.S. Pat. No. 6,594,087).
As another type of a positive-lead type zoom lens, a zoom lens including four lens units having refractive powers of positive, negative, positive and positive in this order from the object side to the image side, and having a zoom ratio on the order of 96 and a photographic field angle at a wide angle end on the order of 62° is also known (U.S. Pat. No. 7,679,837).
Among zoom lenses including five lens units composed of lens units having refractive powers of positive, negative, positive, negative, and positive in this order from the object side to the image side, a zoom lens in which lenses formed of material having anomalous dispersion properties are used in the first lens unit is known (U.S. Pat. No. 7,304,805).
As another example, a zoom lens including five lens units having refractive powers of positive, negative, positive, positive, and positive, or refractive powers of positive, negative, positive, positive, and negative, or refractive powers of positive, negative, positive, negative, positive in these orders from the object side to the image side, having a zoom ratio on the order of 24 and a photographic field angle at a wide angle end on the order of 77° is also known (Japanese Patent Laid-Open No. 2004-117826).
The positive-lead type zoom lens is relatively easy to achieve a high zoom ratio while reducing the size of the entire system. Reduction of the size of the entire system of the positive-lead type zoom lens while maintaining a predetermined zoom ratio is achieved only by reducing the number of lenses while increasing the refractive powers (optical power=inverse of focal length) of the respective lens units which constitute the zoom lens. However, the zoom lens in this configuration suffers from a significant aberrational variation during a zooming operation and has a difficulty in obtaining high optical performance over an entire zoom range.
In addition, in order to secure a certain cut-end thickness in association with increase in refractive powers of the respective lens surfaces, the thickness of the lenses may increase. In particular, the front lens effective diameter is increased, and hence the reduction of the lens system as a whole may become insufficient. Furthermore, if an attempt is made to increase the zoom ratio in the positive-lead type zoom lens, a secondary spectrum of axial chromatic aberration may often be generated in a zoom area on the telephoto side. In the positive-lead type zoom lens, in order to obtain a high optical performance over the entire zoom range while trying to achieve a high zoom ratio, reduction of the chromatic aberration, specifically, the secondary spectrum is important.
In order to reduce the chromatic aberration and the secondary spectrum, it is effective to use a lens formed of a material having low dispersion properties and anomalous dispersion properties at an adequate position in the zoom lens. As regards the chromatic aberration, optimization of the respective lens units which constitute a zoom lens on the basis of material characteristics (Abbe number or partial dispersion ratio) is important.
In particular, in the positive-lead type zoom lens including four lens units or five lens units described above, a reduction of the secondary spectrum on the telephoto side becomes difficult when an attempt is made to achieve a high zoom ratio unless the first lens unit having a positive refractive power is set adequately. Consequently, achievement of a high optical performance over the entire zoom range becomes difficult.
In addition, adequate setting of imaging magnifications of the second and the third lens units and the lens units to be moved during the zooming operation as the refractive power of the third lens unit is important. Without setting these configurations adequately, obtainment of a zoom lens having a wide field angle, a high zoom ratio, and a high optical performance while achieving the reduction in size of the entire system becomes difficult.